docs: def Guo_Sun(dp, voidage, vs, rho, mu, Dt, L=1):
      r'''Calculates pressure drop across a packed bed of spheres using a
      correlation developed in [1]_. This is valid for highly-packed particles 
      at particle/tube diameter ratios between 2 and 3, where a ring packing 
      structure occurs. If a packing ratio is so low, it is important to use this
      model because in some cases its predictions are as low as half those of 
      other models!
  
      .. math::
          f_v = 180 + \left(9.5374\frac{d_p}{D_t} - 2.8054\right)Re_{Erg}^{0.97}
          
      .. math::
          f_v = \frac{\Delta P d_p^2}{\mu v_s L}\frac{\epsilon^3}{(1-\epsilon)^2}
  
      .. math::
          Re_{Erg} = \frac{\rho v_s  d_p}{\mu(1-\epsilon)}
  
      Parameters
      ----------
      dp : float
          Particle diameter of spheres [m]
      voidage : float
          Void fraction of bed packing [-]
      vs : float
          Superficial velocity of the fluid (volumetric flow rate/cross-sectional 
          area)[m/s]
      rho : float
          Density of the fluid [kg/m^3]
      mu : float
          Viscosity of the fluid, [Pa*s]
      Dt : float
          Diameter of the tube, [m]
      L : float, optional
          Length the fluid flows in the packed bed [m]
  
      Returns
      -------
      dP : float
          Pressure drop across the bed [Pa]
  
      Notes
      -----
      Developed with data in the range of:
  
      .. math::
          100 < Re_{m} <33000\\
          2 < d_t/d_p < 3 1\\
          0.476 < \epsilon <0.492
  
      Examples
      --------
      >>> Guo_Sun(dp=14.2E-3, voidage=0.492, vs=0.6, rho=1E3, mu=1E-3, Dt=40.9E-3)
      42019.529911473706
      
      References
      ----------
      .. [1] Guo, Zehua, Zhongning Sun, Nan Zhang, Ming Ding, and Jiaqing Liu. 
         "Pressure Drop in Slender Packed Beds with Novel Packing Arrangement." 
         Powder Technology 321 (November 2017): 286-92.
         doi:10.1016/j.powtec.2017.08.024. 
      '''
      #  2 < D/d < 3, particles in contact with the wall tend to form a highly ordered ring structure. 
      Rem = dp*rho*vs/mu/(1-voidage)
      fv = 180 + (9.5374*dp/Dt - 2.8054)*Rem**0.97
      return fv*(mu*vs*L/dp**2)*(1-voidage)**2/voidage**3